1. Field of the Invention
The present invention relates to arthroplasty and devices and methods for joint replacement and more specifically to a metaphyseal element of a prosthesis and methods of installing and removing such a prosthesis.
2. Background
The use of implantable hip and shoulder prosthesis systems is well known in the prior art. A widely used configuration of such prosthetic devices includes a stem intended for insertion into the prepared end of long bone to which the device is to be fit for joint reconstruction. A head including cooperative joint element is fixed or fixable to the stem. Depending on the particulars of any given design for such a prosthesis, there may be a distinguishable metaphyseal element of the prosthesis that provides a transition between the stem and the head.
Fixation of the stem of the prosthesis within the intramedullary bone has been, as seen in the prior art, achieved press-fitting the stem into the intramedullary bone or in the alternative by cementing the stem of the prosthesis upon insertion into the shaft of the bone. According to the prior art, the stem of a press-fit prosthesis typically has a textured or roughened surface or surface coating. Such a prosthesis achieves initial bony fixation from the hoop stresses and friction associated with the interference fit that occurs as the device is forcibly inserted into the bone. Over time the bone remodels, leading to a change in the mode of fixation. The hoop stresses and interference fit diminish as the bone remodels with time, and eventually the primary mode of fixation becomes fusion between the roughened prosthetic surface and the native bone. The robust bone-to-prosthesis fusion that occurs along the length of roughened or coated stem makes the prosthesis difficult to remove without destroying the bone. Additionally, while this technique may provide good fixation for a period of time, the strong fixation that occurs from bony fusion to the distal aspect of such a prosthesis will later cause stress shielding of the proximal bone, eventually leading to a proximal osteolysis that leaves the bone/stem fit predisposed to loosening and the bone predisposed to fracture over time. Similarly, in an arthroplasty where cement has been used to secure the stem of the prosthesis in the shaft of the bone, fixation of the stem to the bone by cementing can be to such an extent that bone is destroyed while freeing the stem for removal. Removal of a humeral prosthesis in the reverse shoulder arthroplasty setting is made even more difficult because the proximal shell of the humeral prosthesis physically limits the surgeon's ability to apply surgical instruments such as an osteotome to free the prosthesis from the bone in the metaphyseal and diaphyseal regions.
Therefore one object of the present invention is to provide a prosthesis for a joint replacement specifically adapted to achieve secure fixation of the prosthesis to the bone by promoting osseous integration in the metaphysis, while eliminating settling, loosening or rotation of the device with respect to the bone. Another object of the present invention is to provide a press-fit shoulder prosthesis optimized for a bone grafting technique. Because bony in-growth occurs ideally in the metaphysis, another object of the present invention is to provide a shoulder prosthesis including a metaphyseal element adapted to promote osseous integration in the metaphyseal region. Another object of the present invention is to provide a press-fit shoulder prosthesis optimized for a bone grafting technique that is also optimized to increase the surface area in the metaphyseal portion of the prosthesis where bony ingrowth/ongrowth may occur, to promote bony ingrowth against and about the metaphyseal region of a shoulder prosthesis. Another object of the present invention is to provide a press-fit shoulder prosthesis that is optimized for ease of removal with minimal trauma to the bone.
Another object of the present invention is to provide a press-fit shoulder prosthesis that reduces inventory costs insofar as the press-fit shoulder prosthesis may be configured so as to be employed for multiple and various indications including as a primary press-fit prosthetic device, a cemented prosthetic device, or a fracture stem/prosthetic device.